The solution

Idea 1: new properties in .desktop files

This would then be used by the appropriate settings programs to present a list of available applications to the user to choose from.

Idea 2: depend on mime types

Some of these actions have corresponding mime types, and fake/new mime types could be added to the .desktop-files which the appropriate settings programs would use to present a list of available programs to the user.

However, this would not allow the programs to specify in which way they should be called.